Thanks for reaching out to the community! You can set up parent and child companies within your portal by following the instructions here. At this time you can only associate one company to a deal. However, other users have requested this feature to add multiple companies to a deal as you can see here on the Ideas Forum. I would suggest upvoting this post and then commenting on your use case on the post. This way you will be updated with any changes or future releases and it also shows our product team that you are interested as this is where the product team checks for feedback on the tool.
For now, as a workaround, you can associate any company activities on a deal record even if the company/deal objects are not associated here is how to associate activities with records.
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